The present invention relates to an inorganic glassy coating and a method of protecting metal alloy bodies that are used generally in the chemical processing industry, such as components of a thermal-cracking furnace, against coke deposit build-up, corrosion, and carburization. In particular, the invention comprises a potassium silicate composition designed to form a durable protective glass that can adhere to the metal bodies to which it is applied.
Industry has long been concerned about protecting surfaces on metal bodies, particularly iron-based alloys, from exposure to carbon at elevated temperatures and pressures in chemical manufacture and processing environments. Two problems commonly resulting from such exposure to carbon, especially in the petrochemical industry, are carbon build-up on the metal surfaces, commonly referred to as xe2x80x9ccoking,xe2x80x9d and carburization of the metal. Both problems are of particular concern in furnaces and tubing that service the thermal cracking of hydrocarbons such as ethane, propane, butane, naphtha, or gas oil to produce olefins, such as ethylene, propylene, or butenes. The cracking furnace forms the heart of many chemical-manufacturing processes. Often the performance of the cracking furnace will determine the major profit potential of the entire manufacturing process. Hence, it is extremely desirable to maximize the performance of the cracking furnace.
At the center of a thermal cracking process is the pyrolysis furnace. This furnace comprises a fire box through which runs an array of tubing. This array may be a set of straight tubes fed from a manifold, but frequently is a serpentine array of tubing. In either case, the array is composed of lengths of tubes and fittings that may total several hundred meters in length. The array of tubing is heated to a carefully monitored temperature by the fire box. A stream of gaseous feedstock is forced through the heated tubing under pressure and at a high velocity, and the product is quenched as it exits.
For olefin production the feedstock is frequently diluted with steam. The mixture is passed through the tubing array, which is operated at a temperature greater than 650xc2x0 C. During this passage, a carboniferous residue, which is extremely durable and difficult to remove, forms and deposits on the interior walls of the tubing and fittings. The carbon residue appears initially in fibrous form on the walls. This type of filamentous coke grows in long threads, that is believed due to a catalytic reaction with nickel and iron in the metal alloy of the tubes. The fibrous carbon appears to form a mat on the interior walls of the tubes. A second type of amorphous coke forms in the gas phase and plates out from the gas stream. The fibrous carbon mat traps the secondary pyrolitic-coke particles that formed in the gas, resulting in a build-up of a dense, coke deposit on the tube walls. This build up of coke in the tubes dramatically shortens their useful life. The coke deposits clog the tubes after only a couple of weeks use. Consequentially, the chemical processing plants currently must either replace the corroded tubing or decoke the tubes, both of which may entail shutting down the chemical processing, and can cost the industry both valuable time and money to restart plant operations, as will be discussed more fully below.
Generally, when referring to coke deposition, the metal surfaces of the cracking tubes that are in contact with the feed stream and the metal surfaces of the heat exchangers that are in contact with the gaseous effluent are the areas affected. It should be recognized, however, that coke may form on connecting conduits and other metal surfaces which are exposed to hydrocarbons at high temperatures. Therefore, the term xe2x80x9cmetalsxe2x80x9d will be used hereinafter to refer to all metal surfaces in a thermal cracking process which are exposed to hydrocarbons and subject to coking.
The problem of carbon deposits forming on the interior surfaces of furnace tubes during the thermal cracking of hydrocarbons, such as ethylene, is one of long standing. Coking restricts the flow of the gaseous stream of reaction materials. But more importantly, the coke can act as a thermal insulator. Thermal insulating effect of the carbon build-up on the tube walls limits the process efficiency of the furnace to heat the gaseous stream. To maintain through-put of the hydrocarbon stream passing through the furnace, workers are forced to compensate by continuously raising the temperature of the fire box so that sufficient heat penetrates the tubing and maintains a steady temperature. The temperature of the tube outerwall continuously increases. Ultimately, a point is reached when the coke build-up is so severe that the tube skin temperature cannot be raised any further and the fire box and tubes reach temperatures where operations must discontinue or risk damage to the machinery. A shutdown of the furnace coils is then needed to remove the carbon deposit in a procedure called decoking, whereby the coke deposit is burned off. The decoking operation is necessary once every 10 to 90 days, and typically lasts for 24 to 96 hours, for light feed stock furnaces, and considerably longer for heavy feed stock operations.
During the decoking operation, no marketable production takes place, which means a major economic loss. Moreover, another effect of carbon formation on metal tubing in the cracking furnace impacts safety. The decoking process degrades tubes at an accelerated rate, leading to a shorter useful lifetime. In addition to the inefficiencies, discussed above, the formation of coke also leads to accelerated carburization and other forms of corrosion and erosion of the tube innerwall.
Carburization results from the diffusion of carbon into the steel alloy, where the carbon reacts with chromium in the alloy forming brittle carbide phases. The metal alloy loses its original oxidation resistance, thereby becoming susceptible to chemical attack. Over time, this process leads to volume expansion and the metal becomes gradually more and more brittle, accompanied with a consequent loss of mechanical strength and possible cracking. Since such chemical operations are typically conducted under considerable pressure and tensile load, the danger of tube rupture increases. Both tensile load and pressure during the thermal cracking process tend to be relatively constant factors, but as a metal tube is weakened these factors become more significant. Again, it then becomes necessary to shut the operations down and completely rebuild the furnace with new tubing. At normal operating temperatures half of the wall thickness of some steel tube alloys can be carburized in as little as two years of service. More typical tube lifetimes range from 3 to 6 years.
In the past, numerous solutions to the problem of coking have been proposed. One such solution involves producing metal alloys having special compositions. Another proposed solution entails coating the interior wall of the tubing with a silicon containing coating, such as silica, silicon carbide, or silicon nitride. Methods illustrative of this second approach have been disclosed in patents such as U.S. Pat. No. 4,099,990 (Brown et al.) or U.S. Pat. No. 4,410,418 (Kukes et al.).
In still other proposals, the interior wall of the tubing is treated with a chromium and/or an aluminum compound. One such method was developed by Westaim Technologies, Inc., which is disclosed in WO 97/41275. The Westaim method generates surface alloys by depositing onto a metal surface elemental silicon and titanium with at least one of aluminum and chromium, and then heat treating to produce a protective coating. In contrast to this more costly approach, which requires tens of dollars to treat a foot of tubing, and thus, rather commercially impractical, one of our purposes is to produce a more cost-effective coating to extend the useful life of pyrolysis furnaces, by means of substantially a single glassy layer.
A practice of incorporating additives, such as organic sulfur and phosphorus compounds in the feedstock stream, in attempts to passivate the tube metal surfaces, has also been employed in commercial processes. Another solution has proposed applying a glass-ceramic coating to metal to protect the metal surface from being carburized and becoming brittle, as well as, to lessen coking during a thermal cracking process. Table 1 provides in weight percent on an oxide basis as calculated from the precursor glass batch, the composition of several, different glass-ceramics, which have been proposed as protective coatings. Examples 1-6 illustrate alkaline earth metal alumino borates or borosilicates. Examples 7-14 illustrate alkaline earth silicates that may contain minor amounts of alumina or zirconia.
The efficacy of glass-ceramic coatings for the purpose of protection appears to be lower than desired for our purposes. Sections of the coating tended to separate from the metal substrate. This undesirable characteristic, we think is caused by an expansion mismatch between the metal and the glass-ceramic coating. Glass ceramics typically have coefficients of thermal expansion (CTE) in the range of 0-100xc3x9710xe2x88x927/xc2x0 C., which is much lower than the typical CTE of metals used in the refineries that range from 120-180xc3x9710xe2x88x927/xc2x0 C. Although normally considered desirable when dealing with standard enameling, this disparity between the lower CTE of typical glass-ceramics compared to metals creates high compressive stresses in the coating, which under the rather severe conditions inside a thermal cracking furnace during its operation increases the likelihood of spalling, since the furnace undergoes rapid thermal change during a decoking cycle.
Even with this intensive effort, the industry continues to face the problems created by carbon deposits on high temperature metal surfaces. Currently, the metal tubes that are used in the chemical processing industry are protected using a variety of coatings on their interior surfaces. In the extremely corrosive atmosphere and high temperatures (500-1150xc2x0 C.) that are involved in the chemical manufacturing process, however, no satisfactory coating is currently available commercially for our desired purposes. Hence, a new material having the requisite adhesion and durability properties is needed to protect metal surfaces that are subject to the high temperature and stresses of pyrolysis furnaces.
An objective of the present invention is to improve upon the anti-coking technology by turning to glassy coatings having a predominant leucite (K2Oxc2x7Al2O3xc2x74SiO2) crystal phase and formed in situ on the metal surface. Although previous research on dental porcelains had focused on leucite as a potential material that could combine some of the desired protective qualities, dental porcelains are nonetheless less well adapted to be able to withstand the high temperatures and sonic speeds of gas flow conditions required in chemical processing tubes. Dental porcelains typically contain a high percentage of glassy-phase relative to their leucite crystal content. Rather than protect any metal tubing to which they may be applied, the major glassy nature of dental porcelain compositions would tend to cause any coating made from them to be vulnerable to being swept away under the scouring conditions of the high-velocity, chemical gas flow. Moreover, dental porcelains tend not to undergo crystallization or phase changes, but rather, the porcelains retain their essential composition as admixtures of glass powders even after heating and sintering. Thus, their initial high glassy-phase content is retained after firing.
The present invention is directed towards minimizing, if not preventing, carbon deposits from forming on certain metal alloy surfaces, in particular with metals used in reference to pyrolysis furnaces, and protecting the metal surfaces against the detrimental effects of carburization, which typically result from the carbon deposits. An embodiment of the present invention employs as coatings a potassium-silicate glass composition in both a pure glass and filled glass form, each with a coefficient of thermal expansion (CTE) of at least 80xc3x9710xe2x88x927/xc2x0 C. Another more preferred embodiment of the present invention uses high thermal expansion glass and glass-ceramic compositions that exhibit good adhesion to metal articles, particularly those articles used in the chemical processing industry, to prevent process gases from reacting with the metal alloy. It is an intention of the invention to provide a new method of forming in-situ a glassy coating with a CTE of at least 80xc3x9710xe2x88x927/xc2x0 C. (preferably, 120-220xc3x9710xe2x88x927/xc2x0 C.). Produced by means of an operation known as the xe2x80x9creactive-cerammingxe2x80x9d process, the glassy coating has leucite as its predominant crystalline phase.
The inventive glass and glass-ceramic coatings create a protective layer between the exposed metal tube surface and corrosive environment inside. In some experimental trials, little if any coke builds-up in tubes treated with the glassy coating. Any amount of coke that does accumulate does so at a slower rate and only loosely bonds to the glass coated interior, unlike filamentous carbon, making the coke deposits softer and easier to remove than before.